Effects of flue gas constituents on mercury speciation

Beginning with the 1990 Clean Air Act Amendments, there has been considerab le interest in mercury emissions from coal-fired power plants. This past ye ar, the U.S. Environmental Protection Agency (EPA) issued both the Mercury Study Report to Congress and the Study of Hazardous Air Pollutant Emissions from Electric Utility Steam-Generating Units, which make clear that EPA vi ews mercury in the environment as a serious issue and that coal-fired utili ties are a major source of mercury. For the past 4 years, EPRI and the U.S. Department of Energy (DOE) have funded research on mercury measurement, co ntrol, and chemistry at the Energy and Environmental Research Center (EERC) . The primary goal of bench-scale work was to determine what flue gas const ituents affect mercury speciation, specifically how mercury speciation affe cts measurement methods and the ability of mercury sorbents to absorb mercu ry. A bench-scale test rig was designed and built to simulate flue gas cond itions, The baseline simulated flue gas consisted of O-2, CO2, H2O, and N-2 . Other flue gas constituents tested include SO2, HCl, NO, NO2, HF, Cl-2, a nd fly ash. The mercury was delivered to system as either elemental mercury (Hg-0) or mercury(II) chloride (HgCl2) via temperature-controlled permeati on tubes. EERC bench-scale data clearly show that the type of fly ash is im portant in determining mercury speciation in flue gas streams. Not surprisi ngly, there appear to be a number of interactions between various flue gas constituents that affect mercury speciation. Depending on concentration, th ere is clearly an interaction between NO-NO2 and fly ash, and it is possibl e that the interaction may be related to the ratio of NO:NO2. However, it h as been shown that when NO-NO2 is tested without fly ash, there is no conve rsion of Hg-0 to Hg2+. Bench-scale tests clearly show that the chemistry of mercury is very complex and that more research is needed to understand wha t is occurring. However, it is equally clear that the development of effect ive mercury sorbents and the ability to accurately model mercury speciation are dependent on understanding mercury chemistry, thermodynamics, and kine tics. (C) 2000 Published hy Elsevier Science B.V. All rights reserved.